This sign would be slightly more accurate if it read "Caution: Puddles". I mean, yes, when it rains, of course water will be on the road. Normally, however, it runs off the sides and into the ditches or down the curbs and leaves only a wet driving surface. On some roads, the water does not roll off and instead stands directly on the road surface, inviting people to hydroplane and do other fun and interesting automotive acrobatics and necessitating the placement of this admittedly badly-worded sign.
In El Paso, this sign should say, "Oh my fucking god, standing water! Stomp your brakes and guarantee you'll lose traction instead of just letting off the gas and cruising through." Either that, or "Heh heh heh, didn't keep your wiring in good shape, eh? When your car stalls on the other side of this puddle, pull to the side so the other motorists can get around you."